Paper that is thinner and lighter in weight is being increasingly demanded to reduce the weight of printed material that must be shipped and mailed. This trend in the paper industry toward lighter weight printing papers has made it necessary to find a means for maintaining in light weight sheets the optical properties such as opacity normally found in heavier weight papers. To impart opacity to such papers, the paper is filled or loaded with mineral pigments. Unfortunately, the use of conventional mineral pigment fillers such as clay and titanium dioxide in the amounts necessary to obtain the desired optical properties can result in severe deterioration of the strength characteristics of the paper. Various techniques have been proposed for producing light weight paper sheets possessing both sufficient strength and the desired optical properties, however, none of these techniques have truly been successful.
For example, a well known method of increasing tensile strength of paper made from cellulosic pulp is by mechanically refining the pulp prior to papermaking. However, while additional refining increases tensile strength, it invariably reduces the opacity of the resulting paper. Another method of compensating for the loss in strength associated with mineral pigment inclusion is through the use of substantial levels of resins, latex, or other dry strength additives. Such dry strength additives can add substantial raw materials cost to the paper due to the relatively high level of additive required to provide sufficient strength.
It has now been discovered that adding expanded fiber to paper containing mineral pigments increases the effectiveness of such pigments and improves tensile strength simultaneously. That is, paper products containing the combination of expanded fiber and an opacifying mineral pigment exhibit both higher opacity and tensile strength than paper containing only a mineral pigment. Moreover, neither the use of the specific combination of expanded fiber and a mineral pigment, nor the desirable opacity and tensile strength properties of paper structures containing these components appear to have been appreciated heretofore.
Expanded fiber is a substance made from fibrous material having a fibrillar ultrastructure, wherein the fibrous material has been processed in such a way as to cause fibrils to separate from, or become disassociated from, the fibrous material ultrastructure. Alternatively, expanded fiber can be considered as cellulosic fibrous material which has been expanded from a fibrous form to a fibrillar form. Expanded fiber from natural, cellulosic fibers is of particular interest herein.
Cellulosic fibers are multi-component ultrastructures made from cellulose polymers. Lining, pentosans and other components known in the art may also be present. The cellulose polymers are aggregated laterally to form threadlike structures called microfibrils. Microfibrils are reported to have diameters of about 10- 20 nm, and are observable with an electron microscope. Microfibrils frequently exist in the form of small bundles known as macrofibrils. Macrofibrils can be characterized as a plurality of microfibrils which are laterally aggregated to form a threadlike structure which is larger is diameter than a microfibril, but substantially smaller than a cellulosic fiber. In general, a cellulosic fiber is made up of a relatively thin primary wall and a relatively thick secondary wall. The primary wall, a thin, net-like covering located at the outer surface of the fiber, is principally formed from microfibrils. The bulk of the fiber wall, i.e., the secondary wall, is formed from a combination of microfibrils and macrofibrils. See Pulp and Paper Manufacture, Vol. 1, Properties of Fibrous Raw Materials and Their Preparation For Pulping, ed. by Dr. Michael Kocurek, Chapter VI, "Ultrastructure and Chemistry", pp 35- 44, published jointly by Canadian Pulp and Paper Industry (Montreal) and Technical Association of the Pulp and Paper Industry (Atlanta), 3rd ed., 983, incorporated herein by reference. The cellulosic fiber walls constitute the ultrastructure of the cellulosic fiber. Microfibrils and macrofibrils shall hereinafter be collectively referred to as "fibrils." Expanded fiber from cellulosic fibers thus refers to fibrils which have been substantially separated from or disassociated from a cellulosic fiber ultrastructure. Fibrous material in this condition shall hereinafter be referred to as being in "fibrillar" form.
Production of expanded fiber, of any type, from fibrous material having a fibrillar ultrastructure involves expansion of the fibrous material from a primarily fibrous form to, at least, a partially fibrillar form. One method for producing expanded fiber from cellulosic, fibrous material is disclosed in U.S. Pat. No. 4,483,743, Turbak, et al., issued Nov. 20, 1984. Expanded fiber, referred to therein as microfibrillated cellulose, is produced by passing a liquid suspension of cellulose fibers through a small diameter orifice, in which the suspension is subjected to a pressure drop of at least 3000 psig and a high velocity shearing action, followed by a high velocity decelerating impact. Passage of the suspension through the orifice is repeated until a substantially stable suspension is obtained.
A preferred method for producing expanded fiber from cellulosic, fibrous material is disclosed in U.S. Pat. No. 4,761,203, Vinson, issued Aug. 2, 1988, incorporated herein by reference. The expanded fiber referred to therein is produced by a process wherein fibrous material having fibrillar ultrastructure is mechanically fibrillated by impacting fine media against such fibrous material. This process involves the steps of first impacting the fibrous material with a plurality of fine media such that fibrils of the fibrous material are separated from fibrous material ultrastructure; and then separating the fibrous material from the fine media. Such treatment may be implemented with apparatuses known as fine media mills, agitated fine media mills and sand mills. Preferably, a horizontal fine media mill, wherein flow of fibrous material through the fine media mill occurs in a substantially horizontal direction, is utilized. Vertical fine media mills and media mills at angles between horizontal and vertical configurations are also applicable.
Other methods in the paper industry have been proposed to increase the level of fibrillation conventionally observed for pulped, cellulosic fiber. For example, beating and additional refining of pulp in excess of the level conventionally practiced in order to provide a commercially saleable product are well known to increase fibrillation. However, beating and refining as practiced in the cellulose fiber industry are relatively inefficient processes. Large amounts of energy are expended to gain relatively low amounts of fiber expansion and fibrillation. In these processes, the fiber is abraded to form a fiber having a "fuzzy" character, while the fiber walls, and hence the ultrastructure, are retained substantially intact. Beating and refining, generally implemented by abrasion and impacting of suspended fibers by entrapment between a rotor or stator, have been found to be of extremely limited utility for producing expanded fiber due to the prolonged period of fiber treatment necessary to achieve levels of fibrillation significant for the manufacture of expanded fiber. Another disadvantage of fibrillation by conventional beating and refining apparatuses is that a high level of wear would be incurred upon the apparatus surfaces.
The process of adding mineral pigment fillers to papermaking furnishes prior to the formation of the paper sheet is well known in the art. See for example, Smook, Handbook for Pulp & Technologists, pages 204- 207 (1987). In particular, finely divided white mineral pigments are frequently added to papermaking furnishes to improve the optical and physical properties of the sheet. Such white mineral pigments are highly desirable in printing papers where they increase the opacity, raise the brightness, and generally improve the printing properties. The application of these mineral pigments is especially important when opacity is needed at a low basis weight.
The most commonly used papermaking opacifying mineral pigments are clay, calcium carbonate, talc, and titanium dioxide. Clay is the most widely used filler pigment because it is a cheap, plentiful, stable and provides generally good performance. Calcium carbonate is used only in alkaline or neutral systems because of its solubility at lower pH levels. It is available at a higher brightness level than clay and is a better opacifier. Talc is a hydrated magnesium silicate with the approximate formula of H.sub.2 Mg.sub.3 (SiO.sub.3).sub.4. Talc is notable as a "soft" filler, imparting a soft silky feel to the paper product. Titanium dioxide is the brightest and most effective opacifier. Only a fraction as much titanium pigment is needed to produce the same opacity as clay, and this difference is particularly noticeable in paper of low basis weight. Another feature of titanium dioxide filled paper is reduced show through after printing. The high price of this pigment does not permit indiscriminate use, therefore, titanium dioxide is used primarily to produce high-quality and high-priced paper products.
The use of titanium dioxide as well as other mineral pigments result in some undesirable effects, principally a measurable and often significant decrease in the strength of the paper. That is, the price paid for the improvement of the optical properties of paper through the addition of a opacifying mineral pigment is often a significant loss in tensile strength. Accordingly, it would be highly desirable to be able to increase the opacity of paper through the use of conventional mineral pigments without adversely affecting the paper's tensile strength.
It is therefore an object of this invention to provide a high opacity paper structure, containing expanded fiber and an opacifying mineral pigment, which has improved strength properties.
It is a further object of this invention to provide a low basis weight paper structure which has a higher opacity at a particular level of tensile strength relative to paper of the same basis weight which does not contain expanded fiber.
These and other objects are obtained using the present invention, as will be seen from the following disclosure.